The frequency spectrum of a double-sideband modulated signal consists of a carrier at a given frequency (f0), and higher and lower frequency sidebands separated from the carrier by integer multiples of the modulation frequency (f0+fmod, f0−fmod, f0+2fmod, f0−2fmod, etc.). The amount of power contained in these sidebands relative to the carrier is determined by the amplitude of modulation relative to the total power, or modulation depth. In analog systems, the modulation depth is kept small to minimize nonlinearities produced by nonlinear transfer functions in the modulator or receiver. This results in a spectrum with most of the optical power residing in the carrier. In systems that cannot tolerate large optical intensities, it is sometimes advantageous to suppress the large carrier component and amplify the information bearing sidebands for transmission.
In one approach, such as is described in R. Montgomery and R. DeSalvo, “A novel technique for double sideband suppressed carrier modulation of optical fields,” IEEE Photon. Tech. Lett., Vol. 7, no. 4, pp. 434-436 (1995) (“Montgomery et al.”), the suppression of the optical carrier in single channel fiber optic links is attempted where the carrier is removed at the transmitter and a new carrier is injected at the receiver. This approach has also been explored as a method of increasing gain in single octave microwave photonic links.
Another carrier suppression technique is removing the carrier with a band-pass filter, e.g. as described in R. D. Esman. K. J. Williams. “Wideband Efficiency Improvement of Fiber Optic Systems by Carrier Subtraction.” IEEE Photon. Technol. Lett., Vol. 7, no. 2, pp. 218-220 (1995), and M. J. LaGasse. W. Charczenko, M. C. Hamilton, and S. Thaniyavarn, “Optical carrier filtering for high dynamic range fibre optic links,” Electron. Lett., Vol. 30, no. 25, pp. 2157-2158 (1994), and using a low biased Mach-Zehnder modulator, such as is described in Montgomery et al. These two techniques are limited in that they are inherently single carrier implementations. Additionally, removing the carrier with a band-pass filter requires a stable carrier wavelength. Furthermore, the band-pass filter will remove the sidebands at low frequencies, limiting its use for lower frequency modulations. Exploiting stimulated Brillouin scattering (SBS) to remove the carrier has also been demonstrated, e.g. as described in K. J. Williams, R. D. Esman, “Stimulated Brillouin scattering for improvement of microwave fibre-optic link efficiency.” Electron. Lett., Vol. 30, no. 23, pp. 1965-1966 (1994) and in S. Tonda-Goldstein, S. Norcia, D. Dolfi and J.-P. Huignard, “40 dB dynamic enhancement of modulation depth for optically carried microwave signals.” Electron. Lett., Vol. 39, no. 10, pp. 790-792 (2003). This technique requires high initial optical powers and the Brillouin scattering will result in a Doppler shift of the carriers, eliminating the ability to re-inject the carriers in a coherent system.
It would be desirable to provide a system capable of suppressing the large carrier component and amplifying the information bearing sidebands for transmission without these limitations.